Water absorbent materials, such as superabsorbent polymers, can be employed in various applications such as in disposable hygiene articles (i.e. diapers, incontinence articles, feminine hygiene products, airlaids and absorbent dressings), household articles, sealing materials, humectants in agricultural products for soil conditioning, in oil-drilling fluids (i.e. lost-circulation material, fracturing fluids), anti-condensation coatings, in agricultural, horticultural and forestry applications for retaining water in the soil and for the release of water to the roots of plants and trees, in the textile industry, in printing applications, in absorbent paper products, in bandages and surgical pads (i.e. wound dressings), in ore treatments, in concrete products, in pet litter, in water treatment, in food pads (i.e. applications related to the transportation of fresh food and food packaging), in detergents, in fire-fighting gels, in sealing materials, in cloud control, as chemical absorbents for the cleanup of acidic and/or basic aqueous spills including water soluble chemical spills, as polymeric gels for the slow and controlled release of cosmetics and pharmaceuticals (also known as drug delivery systems) and finally in the manufacture of artificial snow. However, the primary use of superabsorbent polymers, also referred as “SAPs”, is in disposable personal hygiene articles. Such products include, in decreasing order of volume of superabsorbent materials used, diapers, training pants, adult incontinence products and feminine hygiene products.
With the development of ultra-thin products, superabsorbent requirements increased. Not only superabsorbents need to absorb large amounts of liquids, but they need also to retain those liquids under stress, swell under pressure and even have a specific gel particle behavior when swollen, as to permit liquids to flow. Among superabsorbents, polyacrylates are widely used today. But current polyacrylates are not biobased, leading to increased carbon footprint, depletion of non-renewable oil reserves and increased vulnerability to energy pricing fluctuations. As an alternative, carboxymethyl cellulose (CMC) is partly biobased and has long been for use as a superabsorbent material.
The major problem of CMC, however, lies in its excessive solubility in water, which causes poor performance properties when deployed as a superabsorbent material. Moreover, the manufacture of CMC typically results in material with an unnecessarily high amount of substitution (carboxylation) per residue, i.e., greater than 0.7 substitutions per residue. Because carboxylation requires the use of petroleum based organic reactants, the excessive carboxylation means increased material cost, a lower degree of renewable matter, and increased carbon footprint. Chatterjee et al.; U.S. Pat. No. 3,731,686; Reid et al.; U.S. Pat. No. 3,379,721 and Ning et al; U.S. Pat. No. 5,247,072 each described means to insolubilize CMC by heat treatment. Acidification has also been described as a means for insolubilization of by CMC Reid et al.; U.S. Pat. No. 3,379,720; Thornton et al.; U.S. Pat. No. 6,765,042 and Kaczmarzyk et al.; U.S. Pat. No. 4,044,766. The major problem with these types of acidification is the use of a liquid solvent as the acid carrier requiring costly liquid handling step, additional energy to dry the solvent. In order to solve those problems, acidic gases had been used to treat CMCs particles, such as described in Ouno et al. U.S. Pat. No. 3,391,135 and Marder et al. U.S. Pat. No. 4,200,737. However, CMCs still have several drawbacks which made them unsuitable as absorbents. One major drawback is that highly absorbent CMCs are very specific to certain types of cellulose fibers meaning that the manufacture of a consistent product requires specific sources of cellulose fibre. Moreover, cellulose fibers from almost all natural sources are occur in a crystalline pattern that must be broken by the carboxymethylation reaction itself, resulting in differential and unpredictable substitution patterns through the cellulose polymer.
Carboxymethyl Starch (CMS) absorbents were far less investigated than CMC. Gross U.S. Pat. No. 5,079,354 and Qin et al. U.S. Pat. No. 5,550,189 described CMS absorbents. Due to water-based reaction inefficiencies or, alternatively, poor performances of dry or solvent synthesis, CMS was only reluctantly explored as a bio based absorbent material. Theodorus et al. NL P 9100249A described CMS extrudates as a possible material for use absorbents. However, the process for manufacture described by Theodorus et al. used excesses of monochloroacetates to generate hydrogen chloride in-situ and resulted in material with significant amounts of residual salts inside the CMS particle, and the particles were uniformly acidified throughout rather than being surface treated as described in more detail herein after. Perhaps due to both the lack of surface treatment and the presence of high amounts of salts, the CMS materials described by Theodorus et al. cannot reach acceptable industry specifications for use as superabsorbent materials for diaper applications, such as having an Absorbency Under Load (AUL) at 0.7 psi of at least 14 g/g and a centrifuge retention capacity (CRC) of at least 18 g/g. More recently, Koutlakis al. US App. 2008/177057A1 described a solvent based treatment of CMS extrudates that resulted in CMS particles with an AULs of at least 14 g/g. However, because those surface treatments were performed in solvent based systems, those processes have similar problems to those described by; Thornton et al.; such as additional liquid handling steps, additional energy costs and particle attrition, which was referred to in that application as “static environment”.
The present disclosure addresses these problems and others, and provides further advantages that one of ordinary skill in the art will readily discern upon understanding the disclosure that follows.